During the past several years a number of methods and apparatus have been developed for combining four or more signals intended for reproduction over a four-loudspeaker system with predetermined amplitude and phase relationships which permit the composite signals to be decoded to recover the four or more original signals with sufficient accuracy that a credible fascimile of the original four-channel program can be reproduced over the four loudspeakers. One such system in commercial use, known as the "SQ" system, includes an encoder utilizing all-pass phase-shift networks and combining networks for producing two composite signals respectively containing as predominant components signals intended for reproduction over loudspeakers positioned at the left front and right front of the listening area and each containing reduced amplitude proportions of both of two signals intended for reproduction over loudspeakers positioned at the left back and right back corners, respectively, of the listening area, the two signals common to both composite signals in one of the composite signals having a substantially constant differential phase-shift angle of 90.degree. relative to the corresponding common signals in the other composite signal. This type of encoder is described in detail in applicant's co-pending application Ser. No. 384,334 filed July 31, 1973 for "Encoders for Quadraphonic Sound System", now Pat. No. 3,890,466. The composite signals produced by this encoding technique not only are amenable to eminently satisfactory decoding but may also be reproduced satisfactorily on conventional stereophonic or monophonic phonographs in the usual manner. The SQ decoder, described in applicant's Pat. No. 3,835,255, derives from the encoded composite signals four output signals respectively containing one of the original input signals as a predominant component.
Although many SQ-encoded records are now available for home reproduction and/or FM broadcasting, the currently available catalog is insufficient for the broadcaster attempting to provide continuous or frequent periodic quadraphonic sevice, and, moreover, it cannot be expected that old time favorite records now available in stereo will soon be reissued in the SQ format. It is desirable, therefore, to be able to modify signals obtainable from conventional stereo records so that when applied to a four-channel decoder and reproduced on four loudspeakers a sound similar to that obtained from a quadraphonic record will result. The realism should be superior to that obtained when conventional stereophonic signals are played through a four-channel encoder, in which case the signals are mainly reproduced over the front channels, with any commonly random or out-of-phase reverberant signals that might be contained in the stereophonic signals transferred in part to the back channels, to create a pleasing ambience which envelopes the listener. This effect, however, is not "quadraphonic enhancement" in the sense that this term is used in the present application.
Although a number of systems have heretofore been proposed for obtaining a four-channel effect from a stereophonic source, examples of which are described in Hafler Pat. No. 3,697,692, and Ito et al Pat. Nos. 3,761,631 and 3,757,047, they do not provide acceptable quadraphonic realism, and do not address the problem of providing quadraphonically-enhanced signals suitable for FM broadcasting and subsequent decoding into four signals for presentation on respective loudspeakers. A more recently proposed system, described in a paper entitled "QS Quadrophonic Synthesizer: What It Does and How It Does It" presented at the 49th Convention of the Audio Engineering Society held on September 9-12, 1974, and distributed as an AES Preprint, involves pre-encoding of the left and right stereo signals preparatory to decoding in a QS vario-matrix four-channel decoder. Encoding is achieved by reverse-phase blending between the left and right stereo signals, a consequence of which is that the total and relative energy content of the encoded signals is altered relative to the energy content of these signals in the stereophonically related signals, resulting in incorrect or low-fidelity sound reproduction.
It is an object of the present invention to provide a method and apparatus for pre-encoding stereophonically related signals derived from a conventional stereo record to produce "encoded" left and right signals which, when played through an SQ decoder and the four output signals therefrom reproduced by respective loudspeakers arranged to define a quadraphonic listening area, the resulting sound field is similar to that obtained from an SQ-encoded quadraphonic record.